This invention relates to an automatic temperature control device for use in an electric appliance such as an electric blanket, electric carpet or the like.
A conventional automatic temperature control device for an electric blanket has been constructed as shown in FIG. 1. In FIG. 1, reference numeral 1 denotes an ac source, 2 a switch, 3 a wire having double functions of heating and temperature sensing and comprising a heater element 4, a sensor 5 and a control element 6.
The wire 3 has a construction such as shown in FIG. 2 in which the control element 6 is wound helically on an insulating core 7. Also, the sensor 5 separates the heater element 4 from the control element 6. Reference numeral 8 denotes an insulating coating. Practically, the wire 3 is arranged in a serpentine fashion inside an electric blanket. The sensor 5 has a negative temperature coefficient of resistance and is usually formed by a plastic thermistor made of a thermosensitive material. This sensor 5 has an impedance Z related to the temperature T as shown in FIG. 3, wherein the impedance Z is a resultant value of a capacitive impedance Z.sub.C and a resistive impedance Z.sub.R, and the temperature sensing property of the sensor 5 is greatly influenced by the capacitive impedance Z.sub.C at low temperatures and by the resistive impedance Z.sub.R at high temperatures. When a DC voltage is applied to a plastic thermistor, the plastic thermistor is polarized and deteriorated to increase its impedance, so that it is necessary to use a plastic thermistor under the application of an ac voltage which is as uniform as possible in its positive and negative polarities.
Then, the conventional example shown in FIG. 1 will be explained in greater detail. In FIG. 1, a switching element 11 (in this example, a semiconductor control device generally known as an SCR is used) is connected in series with the heater element 4 between lines 9 and 10. Similarly, resistors 12 and 13 are connected in series therebetween. Between the junction of the resistors 12 and 13 and a gate of the SCR 11, a variable resistor 14, a resistor 15, the control element 6, a diode 17 and a triggering element 18 are connected in series. Arranged between the control element 6 and the heater element 4 is a capacitor 19. In this automatic temperature control device, during positive half cycles of the ac source voltage in which the line 9 is at a positive potential relative to the line 10, the capacitor 19 is charged at a potential which is determined by the resistance values of the resistors 12 and 13, variable resistor 14, resistor 15 and sensor 5, and when the potential on the capacitor 19 reaches a breakdown voltage of the triggering element 18, a trigger pulse is applied to the gate of the SCR 11 so that the SCR 11 is turned on to supply electric power to the heater element 4. The conduction phase angle of the SCR 11 becomes approximately 0.degree. when the heater element 4 is at low temperature and hence the sensor 5 has a high impedance, thereby supplying maximum electric power to the heater element 4. While, when the temperature of the heater element 4 is high and the impedance of the sensor 5 is low, the conduction phase angle becomes approximately 90.degree. to decrease the supply of electric power to the heater element 4. In this way, the automatic temperature control is carried out. Since the phase angle is changed by the variable resistor 14, the user can obtain a desired blanket temperature by selecting a suitable resistance value for the variable resistor 14.
However, the aforementioned automatic temperature control device has the following disadvantages. Namely, since the circuit comprising the diode 17 is connected in parallel with the sensor 5, the ac voltage applied to the sensor 5 at positive half cycles is different from the ac voltage applied thereto at negative half cycles. Consequently, the sensor 5 is polarized and deteriorated to increase its impedance, which results in a danger such that the controlled temperature shifts to higher temperatures. Further, in the event of a failure of the components used such as short-circuiting of the triggering element 18, the supply of electric power to the heater element 4 becomes uncontrollable still maintaining a maximum amount of electric power supply, thereby causing a danger such that the heater element 4 is overheated. Additionally, a similar danger takes place when the SCR 11 is short-circuited, or a self-triggering failure occurs in the SCR 11 in which the SCR 11 is turned on without being triggered by triggering pulses. Thus, it is possible that these dangerous conditions may cause injury in a worst case situation, since an electric blanket is used by babies or aged persons who cannot push aside an overheated blanket by themselves. Further, the automatic temperature control device employing the phase angle control of an SCR has brought about an unfavourable condition in that noise is generated which causes interference, in particular, with a radio receiver while it is used by a user lying in his bed.